This project is proposed to develop a novel continuous-wave (CW) 1.25um-center-wavelength 250nm-bandwidth low-coherence light source for micron-and cellular-level-resolution optical coherence tomography (OCT). A number of clinical applications of OCT have been demonstrated in a widely diverse range of areas such as ophthalmology, dentistry, cardiology, and gastrointestinal pathologies. Higher resolution imaging may not only provide more accurate/precise diagnosis, but also enable earlier detection and prevention of latent diseases. In addition, it may shorten the trial period of new drug tests. The novel OCT light source proposed herein is a CW amplified-spontaneous-emission (ASE) broadband light source similar to superiuminecent diode yet with about 4 times larger bandwidth. The wider the source bandwidth, the better the axial resolution. For a test sample of 1.6 in refraction index, an axial resolution of 2 um is anticipated. The gain medium is a novel solid-state fiber material called Cr4+ doped nano crystal fiber (CNCF). And it will be fiber-pigtailed and directly pumped by a low-cost multimode laser diode leading to a compact, robust, inexpensive, and fiber-based OCT light source. A pump power of 4 W and a double-clad fiber structure facilitating high-power clad pumping will be employed for producing minimum 3 mW CW exit-fiber power that is comparable with current commercial OCT light sources yet with a much wider bandwidth. To couple a large-core ASE light source into a single-mode fiber (SMF), the output end of CNCF will be fusion spliced with a thermally-diffused expanded core fiber that has a 30-um-diameter core at input end and a 9-um-diameter core at output compatible with a SMF. And a filter will be used to screen any residual pump power. At last, the axial resolution and dynamic range of this source will be evaluated by optical coherence domain reflectometry (OCDR). Relevance: This research would significantly enhance the resolution of optical coherence tomography, which is a real- time non-invasive optical imaging technique increasingly used in the diagnosis of diseases. Higher resolution imaging may not only provide more accurate/precise diagnosis, but also enable earlier detection and prevention of latent diseases. [unreadable] [unreadable] [unreadable]